This thesis is focused on various aspects of particle physics and cosmology from String/M theory. Assuming our universe is a solution of string/M theory, physics below the unication scale is an eective 4D supergravity theory with an abundance of moduli and axions. The phenomenology of moduli and axions in an early universe is studied. We particularly study dark radiation constraints on a generic Axiverse scenario and provide various solutions to it. The simplest solution requires the lightest modulus decays only into its own axion superpartner and this severely constrains the moduli Kahler potential and mass matrix. We also study a model building aspect of string/M theory. It has been shown that a discrete symmetry on a manifold with G2 holonomy combined with symmetry breaking Wilson lines provide a solution to the doublet-triplet splitting problem. We extend the idea to a new class of model based on M theory compactied on a G2 manifold which leads to a novel solution where the colour triplets are decoupled. The models also involves an extra vector-like standard model multiplet to restore gauge unication. We will also discuss the phenomenology of the new light states and the induced R-parity violation. We will also study the prospects of searches from a future generation of colliders. We focus in particular on the search at a 100 TeV collider via the WZ channel. The motivation from string/M theory models leads to the assumption that Higgsinos form the lightest supersymmetric particle. We design simple signal regions for the trilepton channel and nd that neutralinos-charginos could be discovered(excluded) up to 1.1 (1.8) TeV.